This invention relates to improved hydraulic fluids, particularly to high temperature, nonflammable hydraulic fluids.
Many hydraulic fluids commonly used are mineral, naphthenic, or synthetic oils which have been selected primarily on the basis of hydraulic properties, without regard for nonflammability requirements. These fluids tend to be highly flammable and cannot be rendered nonflammable by the use of additives or special processing.
Among the synthetic oils which have acceptable hydraulic properties and which are also commercially available are the chlorotrifluoroethylene-derived oils (hereinafter referred to as "CTFE" oils). These oils are essentially nonflammable due to their high degree of halogenation and can thus be used in hydraulic applications where the non-reactivity of the fluid is an essential requirement. CTFE oils are saturated, low molecular weight oligomers of chlorotrifluoroethylene, typically having about 2 to 10 repeating units in the oligomer chain. The terminal groups of the oligomer chain are generally derived from the catalyst and/or the solvent used in the oligomerization process. The chemical and thermal stability of such CTFE oils is enhanced by chlorination or fluorination of the terminal groups of the oligomer.
CTFE oils are not generally useful by themselves. They do not provide the degree of rust- and corrosion-inhibition provided by hydrocarbon fluids. The viscosity and pressure-viscosity coefficients of unformulated CTFE, at higher temperatures, are much lower than those for hydrocarbon-based hydraulic fluids. This results in lower elastohydrodynamic (EHD) film thickness at the ball/race contact, thereby creating a mixed lubrication regime instead of the desired full separation. Excessive metal-to-metal contact can result in premature failure of critical pump components such as the rolling bearings and splines.
Accordingly, in order to provide a useful CTFE working fluid, it is desirable to incorporate at least a rust-/corrosion-inhibitor and a lubricity additive into the base fluid. Unfortunately, it has been found that many otherwise effective antirust additives, when formulated with lubricity additives, cause the lubricity additive to become ineffective.
Accordingly, it is an object of this invention to provide a CTFE-based working fluid comprising a rust-/corrosion-inhibitor and a lubricity additive.
Other objects and advantages of the invention will be apparent to those skilled in the art.